A cell search procedure is typically initiated by a user terminal upon power on or in preparation for a potential handover. In Evolved Universal Terrestrial Radio Access (E-UTRA) networks, the cell search procedure is based on a synchronization process between a user terminal (or User Equipment, UE) and a base station (or eNodeB) of the E-UTRA network potentially serving the user terminal. The synchronization process involves Primary and Secondary Synchronization Signals (PSSs and SSSs) and provides frequency and symbol synchronization, frame synchronization and cell signature detection.
504 unique Physical Cell Identities (PCIs) can be assigned in an E-UTRA network. The PCIs are grouped into 168 PCI groups, each PCI group comprising 3 PCIs. The space of 504 PCIs is jointly spanned by the PSS and SSS as defined in Section 6.11 the 3rd Generation Partnership Project (3GPP) Technical Specification (TS) 36.211 V.8.9.0 (2010-01). Each SSS additionally carries frame timing information indicative of whether the particular SSS is transmitted in subframe 0 or subframe 5. The timing information is exploited for frame synchronization. Additionally, the SSS is indicative of a particular Cyclic Prefix (CP) configuration employed by the respective eNodeB.
In E-UTRA networks, two different duplex modes are defined. Time Division Duplex (TDD) networks are operated synchronously, which means that the respective cells are aligned with respect to their frame timing. This also implies that synchronization signals from different cells may overlap at least partially in time. Frequency Division Duplex (FDD) networks operate either synchronously or asynchronously. It has been found that even in an asynchronous mode of operation, an at least partial overlap of synchronization signals from different cells may occur.
As a consequence of overlapping synchronization signals from different cells, the cell search procedure will be impaired. It has been observed in E-UTRA networks that cell search procedures become particularly error-prone in case of overlapping synchronization signals from “competing” cells utilizing the same PSS version. As a consequence, it takes more time to detect weaker cells potentially hidden behind a stronger cell. Under unfavourable conditions, the weaker cells may not be detectable at all.